This invention relates generally to a control system for regulating heating and cooling in the perimeter area of a building, and more particularly has reference to a control system that regulates according to the accumulated direction and magnitude of heat flow through the exterior building envelope and the instantaneous deviation of the interior temperature from a desired set point temperature.
The central air handling system of a building is normally regulated in accordance with temperature measurements taken in the core of the building. The resulting temperatures produced in the space are usually comfortable for persons located in the core, but are frequently uncomfortable for persons located near the perimeter of the building. This is because the effects of heat loss or heat gain through the exterior walls of the building are more noticeable to persons in the perimeter areas.
Accordingly, many buildings have been provided with perimeter control systems that attempt to bring the temperature of the building perimeter into closer correspondence with the temperature of the building core. Some of these systems use a heat pump positioned adjacent the exterior walls. Others use a constant temperature, variable air volume or variable temperature, constant volume air handling system mounted in a perimeter region of the ceiling. Those units are normally used in combination with a baseboard heater, a heating coil in the duct, or a fan which recirculates warm air from a ceiling plenum.
Many of the perimeter control systems operate in accordance with control signals provided by temperature sensors. Some use a single sensor located on an interior surface of an exterior wall. Other use a plurality of sensors, one being located to sense the air temperature at the building core, another being located to sense outside air temperature, another located to sense the temperature adjacent the perimeter control system, and others located to respond to the radiant heat of the sun.
Unfortunately, the thermal energy requirements in the perimeter area of the building are not merely a function of instantaneous temperatures, but also are affected by heat loss or heat gain through the exterior walls of the building. For example, a change in the relative position of the sun or a rapid change in wind velocity can have an immediate impact, on the thermal energy requirements in a perimeter area adjacent a window even though there may be no immediate change in temperature in that area. Perimeter control systems regulated by air temperature sensors alone can be slow to react to these types of heat flow changes, resulting in inaccuracies and inefficiencies in the thermal management of the space.
This problem was addressed by the perimeter control system described in U.S. Pat. No. 4,274,475 which used a heat flow sensor mounted on a window or some other thermally conductive element of the building exposed to outside weather conditions. The sensor measured the instantaneous net heat flow through the building element to generate a signal which was proportional to the rate of net heat flow taking place at any given time. The signal controlled the system so that the heating or cooling was proportional to the instantaneous rate of heat loss or heat gain through the building element. Neither inside nor outside air temperature measurements were required.
The system regulated by instantaneous heat flow measurements was a significant advance over earlier systems which were regulated by temperature measurements, but it comprehended the use of a heater alone or a cooler alone. Moreover, it was used successfully only with conventional, i.e., non-compressor type, heating and cooling devices which did not have to be shut down for a certain length of time between cycles.
The heat flow properties of a space are determined by its overall physical characteristics such as window type, window size, wall area, and the like. A perimeter control system that introduces thermal energy into a space in response to signals produced by a heat flow sensor mounted on a selected building element must therefore be adapted to take account of those factors. During morning warm-up, information regarding the thermal capacity of the building was estimated or empirically developed in order to optimize the warm-up time. A manually selected gain factor was used to set up the required relationship of heat loss to make-up heat and match the system to the building characteristics. Any errors in the assessment or in the subsequent estimation of the gain could result in less than optimal thermal management of the space.
Once set, the gain of the system regulated by instantaneous heat flow measurements remained fixed throughout operation. The system thus introduced thermal energy into the space at a rate which was directly proportional to the measurement of instantaneous heat flow at any given time irrespective of the deviation between the actual temperature of the space and the desired set point temperature. If the delivered heat did not exactly match losses, as was the case where the building leaked air at the perimeter, the space temperature would drift up or down. A simple on/off temperature override was used to turn the perimeter heat full "on" or full "off" for a brief time to bring the system within the desired limits of space temperature.
It is conventional practice to shut off the central heating and cooling system at night when the space is unoccupied. A recovery routine is then initiated early in the morning to change the temperature of the space from the night setback level to the desired occupancy temperature in time for the return of the occupants. During the night setback period, the core air handling system is shut down and the perimeter system is used to hold the building at night setback temperature and to warm the space in preparation for occupancy. It is desirable to change the space temperature in a linear fashion over as short a period of time as possible to minimize energy consumption. However, during this period when the building is being brought to occupancy temperature, the heating and cooling system needs sufficient energy in reserve to respond to emergency conditions such as a sudden cold snap. The perimeter control system which was regulated by instantaneous heat flow measurements when operating in the normal occupancy mode responded to the deviation between the night setback temperature and the desired occupancy temperature when operating in the morning recovery mode to change the temperature of the space in a non-linear fashion.
A need thus exists for a heat flow regulated perimeter control system which has automatic switching between heating and cooling, including control of heat pumps and air conditioning units, which is self-adapting to the thermal properties of the space, and which has optimized performance in both the normal and morning recovery modes of operation.